With the growth of optical communication systems, the need for laser sources operating at well defined spectral frequencies or wavelengths has arisen. Wavelength Division Multiplexing (WDM) systems employ several laser systems, each of them modulated at a unique working wavelength. The modulated optical signals are subsequently multiplexed, and spectrally separated channels are combined and delivered through one or more optical fibers. At a receiver end the channels are separated by way of their wavelengths being demultiplexed and routed to individual detectors.
Although WDM systems significantly increase the capacity of a single optical fiber, it comes at a price; control of the wavelength accuracy of each individual laser source must be maintained. Any significant wavelength drift of any channel will cause signal degradation of that channel, or perhaps other adjacent channels at the receiver end. The wavelengths of semiconductor laser sources used in optical WDM systems should be controlled to within a fraction of the channel spacing defined by the ITU grid.
A wavelength monitor (WM) is commonly used in conjunction with laser systems to monitor changes in the wavelength or frequency of the emitted radiation. The WM can be used as an independent device, or can be combined with a laser system forming a wavelength locker (WL) to stabilize and maintain the operational wavelength of one or more lasers by detecting the relative change in the operating wavelength, then generating a feedback signal proportional to the deviation of the working wavelength from its nominal value. The feedback signal is further used to adjust the operating wavelength until the feedback signal is reduced to an acceptable level.
Different wavelength monitoring and locking technologies have been used in the past. One type of wavelength locker is based on thin-film interference filters as disclosed in U.S. Pat. Nos. 4,309,671; 6,122,301; 6,144,025; 6,411,634. A common deficiency of a filter-based approach is that a plurality of filters are required, wherein each filter can be used for locking a relatively small number of neighboring ITU channels. To cover a broad spectral range, such as C and L telecommunication bands, a large inventory of different filters is required; this increases the cost, inventory required, and manufacturing complexity. The problem becomes even more difficult with reduction in channel spacing due to increased filter fabrication cost and complexity.
Another commonly used type of WM employs Fabry-Perot etalons and is based on multi-beam interference, as disclosed in the following U.S. Pat. Nos. 5,825,792; 6,005,995 and US Patent Application US 2003/0063871, all incorporated herein by reference. The thickness of an etalon and the refractive index of the material define the free spectral range (FSR) that corresponds to the spacing of wavelength locked channels. The etalon surface reflectivities should be controlled to achieve a required finesse that defines desired amount of wavelength discrimination.
There are several problems associated with etalon-based wavelength monitors. An etalon-based WM in a front-facet configuration usually requires a beam splitter or tap to redirect part of the output beam onto the WL. This leads to increase in cost, complexity and packaging spatial requirements of the laser system.
To achieve an etalon response function with a desired contrast, operation at a nearly normal incidence angle is required. Because the set point is positioned in the middle of the etalon amplitude modulation response curve, a significant amount of light is reflected and can potentially be coupled back to the laser source. If not rejected, that light will cause performance degradation. To reject the fed-back light, an optical isolator is positioned between the laser diode (LD) and the WL, increasing the product cost, package complexity and spatial requirements.
A third group of wavelength lockers employs wavelength-selective devices based on two-beam interference, such as a Mach-Zehnder interferometer (see for example U.S. Pat. No. 6,549,548). This type of WL has a sinusoidal spectral response and, for a given ITU channel spacing, exceeds the capture range and the contrast of the etalon-based WL counterpart. At the same time, the WL disclosed in U.S. Pat. No. 6,549,548 is based on a complex birefringent waveplate filter system that uses several components and requires precise fabrication and assembly. It also requires a beam-splitter to redirect part of the output beam onto the WL. This type of WL is expensive, complex and adds significant cost to the laser system as a whole.
It would be, therefore, desirable to provide a simple WL device that overcomes the disadvantages of the existing wavelength lockers while providing inexpensive fabrication and reduced packaging complexity.